The theory, feasibility and accuracy of an ultrasonic method of estimating fetal weight

A systematic review of the ultrasound estimation of fetal weight

OBJECTIVES

The range and use of ultrasound fetal measurements have gradually been extended. Measurements have been combined to estimate fetal weight by mathematically based non-linear regression analysis or physically based volumetric methods. Fetal weight estimation is inaccurate, with poor sensitivity for prediction of fetal compromise. Several authors have shown the unacceptable level of intra- and interobserver variability in fetal measurement and the impact of errors on growth assessment. The aims of this study were to review the available methods and possible sources of inaccuracy.

METHODS

Four databases were searched for studies comparing ultrasound estimated fetal weight (EFW) with birth weight. Studies meeting the inclusion criteria evaluated 11 different methods. Errors were graphically summarized.

RESULTS

No consistently superior method has emerged. Volumetric methods provide some theoretical advantages. Random errors are large and must be reduced if clinical errors are to be avoided.

CONCLUSIONS

The accuracy of EFW is compromised by large intra- and interobserver variability. Efforts must be made to minimize this variability if EFW is to be clinically useful. This may be achieved through averaging of multiple measurements, improvements in image quality, uniform calibration of equipment, careful design and refinement of measurement methods, acknowledgment that there is a long learning curve, and regular audit of measurement quality. Further work to improve the universal validity and accuracy of fetal weight estimation formulae is also required.

A simple estimated fetal weight equation for fetuses between 24 and 34 weeks of gestation

OBJECTIVE

To develop a mathematical equation that is simple, accurate and easy to use when applied to low-birth weight or preterm fetuses (< 35 weeks) and to assess previous normal ultrasonic fetal weight curves and make a comparison with normal fetal delivery weight curves.

METHOD

In a large teaching hospital, 269 pregnant mothers were identified by the criteria of normalities, such as: well known LMP, regular menstrual cycles, no use of OCP for the last 3 months, no smoking and no history of diabetes. Birth-weight measurements (adjusted for maternal age, baby's sex, parity and week of gestation) were taken immediately after birth.

RESULTS

Mean gestational age and mean birth' weight + S.D. were 29.5 + 3.02 weeks and 1530.238 237.856 g, respectively. With the aid of a scientific calculator the data were analyzed and a simple regression equation has been derived: EFW (kg) = 0.17 (G.A. - 20), S.D. - 235 g (Honarvar's Formula 1).

CONCLUSION

For estimating weights of preterm or low-birth weight fetuses of less than 2500 g, this simple equation appears to be clinically reliable and easy to use and suggests that previous normal ultrasonic fetal weight curves may underestimate or overestimate normal fetal delivery weight between the 24th and 34th week of gestation. Our formula approximates actual birth weight better and recommends Ott's ultrasonic weight curve for Iranian population.

Intrapartum ultrasonographic estimates of fetal weight by the house staff

In spite of the widespread use of ultrasonographic estimates of fetal weight, a paucity of data exists with regard to its use in patients who are in labor. The purpose of this study was to evaluate the accuracy of ultrasonographic estimates of fetal weight in a busy labor and delivery suite by the house staff. Measurements of biparietal diameter, abdominal circumference, and femur length were prospectively obtained in 109 patients in labor in whom this information was expected to be contributory in making delivery plans. All patients were delivered within 48 hours of ultrasonographic evaluation. Measurements of abdominal circumference were obtained in all cases. Biparietal diameter and femur length were obtained in 85% and 92% of cases, respectively. Overall, the mean absolute errors were 9.3% and 9.2% for estimated fetal weight by biparietal diameter/abdominal circumference and abdominal circumference/femur length ratios, respectively. Estimated fetal weight by biparietal/diameter/abdominal circumference ratio was not significantly different from that by femur length/abdominal circumference ratio. In conclusion, the accuracy of intrapartum estimates of fetal weight performed by the house staff in a busy labor and delivery unit is comparable to that reported for estimates obtained during the antepartum period by professional users of ultrasonography in a controlled setting.

A new method for fetal weight estimation using real-time ultrasound

A method for estimating fetal weight by calculating an approximate fetal volume from abdominal area, head area and femur length has been developed. Birthweights were calculated within 48 h before delivery for 434 patients, using this method and two other methods already in widespread use. Statistical analysis of the three sets of predictions gave standard deviations of 297 g for the new method, compared with 323 g and 342 g for the other methods. Further analysis of the data showed that estimates closest to the true birthweight were obtained in 41% of cases by using the new method, and in 33% and 26% of cases using the other methods.

Estimation of fetal weight in the third trimester by ultrasound

A method to estimate the intrauterine fetal weight by use of ultrasound measurements of the fetal biparietal diameter (BPD) and the abdominal diameter (AD) is presented. From a consecutive series of single pregnancies the 238 pregnancies (3% of the hospital population) with ultrasound measurements obtained within 4 days before delivery were used in the estimation of birth weight. In addition, the estimated formula was applied on a test material consisting of 100 similarly selected pregnancies from the same hospital. To evaluate the expected selection effects, the birth weight for gestational age in the study group was compared with birth weight for gestational age in the total population. The weight could be estimated as 0.0351 X AD1.65 X BPD0.69 X exp(0.00196 X gestational age). Thus, the actual birth weight was within 83-120% of the estimated weight (95% prediction limits), with a residual coefficient of variation of about 9%. The gestational age could be omitted without major influence on the weight prediction. When applying the formula on the test material, 70% of the actual weights deviated less than 10% from the estimated fetal weight, but a tendency towards a slight overestimation of the weight for light for gestational age infants was found.

Risk in the vaginal delivery of the large fetus

The problem of the excessively weighted fetus was studied. For this purpose, over a period of 2 years, each delivery with fetal weight of 4,500 g or more was included in the study. The history and examination findings of the present pregnancy were reviewed following delivery. The postpartum outcome and later complications in both mothers and infants were investigated prospectively. A total of 82 deliveries of large babies were included in the study and the outcome was compared with that of parallel deliveries of babies within the normal weight range (2,500-4,000 g). Factors frequently associated with excessive fetal weight were identified. A relatively high perinatal mortality and morbidity relating to vaginal delivery were observed among the large infants. In all those cases who developed delay in the second stage of labour, Caesarean section is proposed when macrosomia is clinically suspected and confirmed by ultrasonic assessment. Moreover elective Caesarean section should be considered before labour, when macrosomia is associated with hyperglycaemia or other pathology.

Longitudinal studies of fetal growth using volume parameters determined with ultrasound

To explore the use of three-dimensional parameters in characterizing fetal growth with ultrasound, the accuracy of volume measurements and the ability to make such measurements during pregnancy have been investigated. Total fetal volumes (TOTV) determined with ultrasound in utero were compared to volume measurements obtained by hydrostatic weighing following induced abortion at 19.4 (+/- 1.8 SD) weeks, menstrual age (MA), and to those calculated from birth weights and density values for term fetuses [39.3 (+/- 0.8 SD) weeks, MA]. This study indicated that ultrasound measurements underestimate the fetal volume [-9.8 (+/- 9.5 SD)% at 19 weeks and -19.0 (+/- 9.3 SD)% at 39 weeks]. To assess the possibility of obtaining growth curves for volume parameters, 20 normal fetuses [based on crown-rump length (CRL), biparietal diameter (BPD), head circumference (HC) and abdominal circumference (AC) growth curves, birth weight (WT), crown-heel length (CHL) and postnatal examination] with known dates of conception were studied. Head volume (HV), thoracic volume (TV), abdominal volume (AV), limb volume (LV) and TOTV were measured at 2- to 3-week intervals during the third trimester. Mathematical modeling indicated that the linear-cubic (LC) model was optimal for HV, TV, AV, and TOTV (R2: 97.3%, 97.2%, 98.1% and 99.4%) while the linear-quadratic (L-Q) model was optimal for LV (R2: 98.4%). Significant individual variation was suggested by the high coefficients of variation (CV) obtained for the optimal model coefficients. Inspection of individual growth curves revealed the presence of three types, "linear," "concave," and "convex" with the "concave" type predominating. The variability was less than that seen in the groups as a whole for most subgroups but was still greater than that observed with linear parameters. These results indicated that although volume growth curves can be obtained, their individual variability is significant and thus more fetuses must be studied before standard curves can be defined.

Fetal weight estimation from ultrasonic three-dimensional head and trunk reconstructions: evaluation in vitro

The theoretical usefulness of volume as a predictor of fetal weight was assessed on 25 dead neonates with weight ranges between 364 and 3,650 gm. The correlation between volume, measured by water displacement, and weight was r = 0.999, with a standard error of 37 gm. A method is described for using volume, calculated from three-dimensional ultrasonic head and trunk reconstructions, to predict fetal weight. For the dead neonates, the correlation between measured weight and calculated head plus trunk volume was r = 0.985, with a standard error of 190 gm. The regression coefficient was 1.73, thus indicating that head and trunk volumes underestimate weight, as might be expected since limbs were not included. We conclude from this "ideal" situation that volume measurements will eventually provide the best estimates of fetal weight, and that three-dimensional head trunk reconstructions are reasonably accurate measures of overall volume.

The use of ultrasound in the assessment of normal fetal growth: a review

This review presents the basic principles underlying the quantitative assessment of normal fetal growth using ultrasound. Data on fetal growth derived from measurements of body, head, chest, and abdominal dimensions, are summarized, compared, and evaluated. Fetal volume and weight determination are also discussed. Most of the data evaluated were obtained in cross-sectional studies, but we present the preliminary results of our longitudinal study of fetal growth for comparison.

An improved method of fetal weight estimation using ultrasound measurements of fetal abdominal circumference

A simple and accurate method is described for estimating fetal weight from a single fetal abdomen circumference measurement at the level of the umbilical vein. The abdominal circumference was converted into a weight centile for the maturity at the time of measurement so that the weight at delivery could be predicted. This method was tested on a population of fetuses presenting by the breech near term. Although the error between actual and predicted weights was 194 g (1 SD) for all the patients, for a large well-defined subgroup the error was much smaller, being only 104 g (1 SD).

Estimation of fetal weight from ultrasonic measurements

Ultrasonic measurements were made on 65 fetuses within 48 hours of delivery. Multiple regression analysis of birth weight and the natural logarithm of birth weight against several measured variables were obtained. The formula giving the best correlation was a polynomial regression of the natural logarithm of birth weight vs. trunk circumference, circumference, and a long axis measurement. The correlation was improved by excluding the first 15 patients but was not improved further by excluding the next 15. The best correlation was 0.944, giving a predicted birth weight error of +/- 103 Gm. (1 S.D.).